Biosynthesis

The vast majority of animals and plants are able to synthesize their own vitamin C, through a sequence of four enzyme-driven steps, which convert glucose to vitamin C.[5] The glucose needed to produce ascorbate in the liver (in mammals and perching birds) is extracted from glycogen; ascorbate synthesis is a glycogenolysis-dependent process.[21] In reptiles and birds the biosynthesis is carried out in the kidneys.

Among the animals that have lost the ability to synthesise vitamin C are simians, guinea pigs, the red-vented bulbul,and fruit-eating bats.[5] Most notably, humans have no capability to manufacture vitamin C. The cause of this phenomenon is that the last enzyme in the synthesis process, L-gulonolactone oxidase, cannot be made by the listed animals because the gene for this enzyme, Pseudogene ΨGULO, is defective.[22] The mutation has not been lethal because vitamin C is abundant in their food sources, with many of these species' natural diets consisting largely of fruit.

It has been noted that the loss of the ability to synthesize ascorbate strikingly parallels the evolutionary loss of the ability to break down uric acid. Uric acid and ascorbate are both strong reducing agents. This has led to the suggestion that in higher primates, uric acid has taken over some of the functions of ascorbate.[24] Ascorbic acid can be oxidised (broken down) in the human body by the enzyme ascorbic acid oxidase.

An adult goat, a typical example of a vitamin C-producing animal, will manufacture more than 13,000 mg of vitamin C per day in normal health and the biosynthesis will increase "many fold under stress".[25] Trauma or injury has also been demonstrated to also use up large quantities of vitamin C in humans.[26]
Some microorganisms such as the yeast Saccharomyces cerevisiae have been shown to be able to synthesize vitamin C from simple sugars.[27][28]

Deficiency

Scurvy is an avitaminosis resulting from lack of vitamin C, as without this vitamin, the synthesised collagen is too unstable to meet its function. Scurvy leads to the formation of liver spots on the skin, spongy gums, and bleeding from all mucous membranes. The spots are most abundant on the thighs and legs, and a person with the ailment looks pale, feels depressed, and is partially immobilized. In advanced scurvy there are open, suppurating wounds and loss of teeth and, eventually, death. The human body can store only a certain amount of vitamin C.,[29] and so the body soon depletes itself if fresh supplies are not consumed.

Smoking cigarettes has a negative correlation to the amount of vitamin c in the blood stream. The relative amounts of vitamin c drop with the increased amount of cigarettes smoked. [30]

History of human understanding

The need to include fresh plant food or raw animal flesh in the diet to prevent disease was known from ancient times. Native peoples living in marginal areas incorporated this into their medicinal lore. For example, spruce needles were used in temperate zones in infusions, or the leaves from species of drought-resistant trees in desert areas. In 1536, the French explorer Jacques Cartier, exploring the St. Lawrence River, used the local natives' knowledge to save his men who were dying of scurvy. He boiled the needles of the arbor vitae tree to make a tea that was later shown to contain 50 mg of vitamin C per 100 grams.[31][32]

Throughout history, the benefit of plant food to survive long sea voyages has been occasionally recommended by authorities. John Woodall, the first appointed surgeon to the British East India Company, recommended the preventive and curative use of lemon juice in his book "The Surgeon's Mate", in 1617. The Dutch writer, Johann Bachstrom, in 1734, gave the firm opinion that "scurvy is solely owing to a total abstinence from fresh vegetable food, and greens; which is alone the primary cause of the disease."

While the earliest documented case of scurvy was described by Hippocrates around the year 400 BC, the first attempt to give scientific basis for the cause of this disease was by a ship's surgeon in the British Royal Navy, James Lind. Scurvy was common among those with poor access to fresh fruit and vegetables, such as remote, isolated sailors and soldiers. While at sea in May 1747, Lind provided some crew members with two oranges and one lemon per day, in addition to normal rations, while others continued on cider, vinegar, sulfuric acid or seawater, along with their normal rations. In the history of science this is considered to be the first occurrence of a controlled experiment comparing results on two populations of a factor applied to one group only with all other factors the same. The results conclusively showed that citrus fruits prevented the disease. Lind published his work in 1753 in his Treatise on the Scurvy.

Citrus fruits were one of the first sources of vitamin C available to ship's surgeons.

Lind's work was slow to be noticed, partly because he gave conflicting evidence within the book, and partly because the British admiralty saw care for the well-being of crews as a sign of weakness. In addition, fresh fruit was very expensive to keep on board, whereas boiling it down to juice allowed easy storage but destroyed the vitamin (especially if boiled in copper kettles[33]). Ship captains assumed wrongly that Lind's suggestions didn't work because those juices failed to cure scurvy.

It was 1795 before the British navy adopted lemons or lime as standard issue at sea. Limes were more popular as they could be found in British West Indian Colonies, unlike lemons which weren't found in British Dominions, and were therefore more expensive. This practice led to the American use of the nickname "limey" to refer to the British. Captain James Cook had previously demonstrated and proven the principle of the advantages of fresh and preserved foods, such as sauerkraut, by taking his crews to the Hawaiian Islands and beyond without losing any of his men to scurvy. For this otherwise unheard of feat, the British Admiralty awarded him a medal.

The name "antiscorbutic" was used in the eighteenth and nineteenth centuries as general term for those foods known to prevent scurvy, even though there was no understanding of the reason for this. These foods included but were not limited to: lemons, limes, and oranges; sauerkraut, cabbage, malt, and portable soup.

In 1907, Axel Holst and Theodor Frølich, two Norwegian physicians studying beriberi contracted aboard ship's crews in the Norwegian Fishing Fleet, wanted a small test mammal to substitute for the pigeons they used. They fed guinea pigs their test diet, which had earlier produced beriberi in their pigeons, and were surprised when scurvy resulted instead. Until that time scurvy had not been observed in any organism apart from humans, and had been considered an exclusively human disease.

Discovery of ascorbic acid

Albert Szent-Györgyi, pictured here in 1948, was awarded the 1937 Nobel Prize in Medicine for the discovery of vitamin C

In 1912, the Polish-American biochemist Casimir Funk, while researching deficiency diseases, developed the concept of vitamins to refer to the nutrients which are essential to health. Then, from 1928 to 1933, the Hungarian research team of Joseph L Svirbely and Albert Szent-Györgyi and, independently, the American Charles Glen King, first isolated vitamin C and showed it to be ascorbic acid. For this, Szent-Györgyi was awarded the 1937 Nobel Prize in Medicine.[34]

In 1928 the Arctic anthropologist Vilhjalmur Stefansson attempted to prove his theory of how the Eskimos are able to avoid scurvy with almost no plant food in their diet, despite the disease striking European Arctic explorers living on similar high-meat diets. Stefansson theorised that the natives get their vitamin C from fresh meat that is minimally cooked. Starting in February 1928, for one year he and a colleague lived on an exclusively minimally-cooked meat diet while under medical supervision; they remained healthy.

Between 1933 and 1934, the British chemists Sir Walter Norman Haworth and Sir Edmund Hirst and, independently, the Polish chemist Tadeus Reichstein, succeeded in synthesizing the vitamin, the first to be artificially produced. This made possible the cheap mass-production of vitamin C. Only Haworth was awarded the 1937 Nobel Prize in Chemistry for this work, but the process for vitamin C retained Reichstein's name.

In 1934 Hoffmann–La Roche became the first pharmaceutical company to mass-produce synthetic vitamin C, under the brand name of Redoxon.

In 1957 the American J.J. Burns showed that the reason some mammals were susceptible to scurvy was the inability of their liver to produce the active enzymeL-gulonolactone oxidase, which is the last of the chain of four enzymes which synthesize vitamin C.[35][36] American biochemist Irwin Stone was the first to exploit vitamin C for its food preservative properties. He later developed the theory that humans possess a mutated form of the L-gulonolactone oxidase coding gene.

Daily requirements

The North American Dietary Reference Intake recommends 90 milligrams per day and no more than 2 grams per day (2000 milligrams per day).[37] Other related species sharing the same inability to produce vitamin C and requiring exogenous vitamin C consume 20 to 80 times this reference intake.[38][39] There is continuing debate within the scientific community over the best dose schedule (the amount and frequency of intake) of vitamin C for maintaining optimal health in humans.[40] It is generally agreed that a balanced diet without supplementation contains enough vitamin C to prevent scurvy in an average healthy adult, while those who are pregnant, smoke tobacco, or are under stress require slightly more.[37]

High doses (thousands of milligrams) may result in diarrhea. Proponents of alternative medicine (specifically orthomolecular medicine)[41] claim the onset of diarrhea to be an indication of where the body’s true vitamin C requirement lies. Both Cathcart[41] and Cameron have hypothesized that very sick patients with cancer or influenza do not display any evidence of diarrhea at all until ascorbate intake reaches levels as high as 200 grams (nearly half a pound).

Vitamin C as a macronutrient

There is a strong advocacy movement for large doses of vitamin C, promoting a great deal of added benefits. Drawing on a wide, [48] but still inconclusive, body of evidence as to the benefits beyond those dosages recommended in the Dietary Reference Intakes, many pro-vitamin C organizations promote usage levels well beyond the current Dietary Reference Intake. The movement is led by scientists and doctors such as Robert Cathcart, Ewan Cameron, Steve Hickey, Irwin Stone and the twice Nobel Prize laureate Linus Pauling and the more controversial Matthias Rath. There is some scientific literature critical of governmental agency dose recommendations.[40][49] The biological halflife for vitamin C is fairly short, about 30 minutes in blood plasma, a fact which high dose advocates say that mainstream researchers have failed to take into account. Researchers at the National Institutes of Health decided upon the current RDA based upon tests conducted 12 hours (24 half lives) after consumption.

Evolutionary rationales

Humans carry a mutated and ineffective form of the gene required by all mammals for manufacturing the fourth of the four enzymes that manufacture vitamin C.[50] The inability to produce vitamin C, hypoascorbemia, is, according to the Online Mendeleian Inheritance in Man database, a "public" inborn error of metabolism.

The gene, Pseudogene ΨGULO, lost its function millions of years ago, when the anthropoids branched out.[51] In humans, the three functional enzymes continue to produce the precursors to vitamin C, but the process is incomplete; these enzymes ultimately undergo proteolytic degradation. Stone[52] and Pauling[39] calculated, based on the diet of our primate cousins[38] (similar to what our common descendants are likely to have consumed when the gene mutated), that the optimum daily requirement of vitamin C is around 2,300 milligrams for a human requiring 2,500 kcal a day.

The established RDA has been criticized by Pauling to be one that will prevent acutescurvy, and is not necessarily the dosage for optimal health.[47]

Therapeutic uses

Since its discovery vitamin C has been considered by some enthusiastic proponents a "universal panacea", although this led to suspicions by others of it being over-hyped.[53] Other proponents of high dose vitamin C consider that if it is given "in the right form, with the proper technique, in frequent enough doses, in high enough doses, along with certain additional agents and for a long enough period of time,"[54] it can prevent and, in many cases, cure, a wide range of common and/or lethal diseases, notably the common cold and heart disease,[55] although the NIH considers there to be "fair scientific evidence against this use."[56] Some proponents issued controversial statements involving it being a cure for AIDS,[57]bird flu, and SARS.[58][59][60]

Probably the most controversial issue, the putative role of ascorbate in the management of AIDS, is still unresolved, more than 16 years after the landmark study published in the Proceedings of National Academy of Sciences (USA) showing that non toxic doses of ascorbate suppress HIV replication in vitro.[61] Other studies expanded on those results, but still, no large scale trials have yet been conducted.[62][63][64]

In an animal model of lead intoxication, vitamin C demonstrated "protective effects" on lead-induced nerve and muscle abnormalities[65] In smokers, blood lead levels declined by an average of 81% when supplemented with 1000 mg of vitamin C, while 200 mg were ineffective, suggesting that vitamin C supplements may be an "economical and convenient" approach to reduce lead levels in the blood.[66] The Journal of the American Medical Association published a study which concluded, based on an analysis of blood lead levels in the subjects of the Third National Health and Nutrition Examination Survey, that the independent, inverse relationship between lead levels and vitamin C in the blood, if causal, would "have public health implications for control of lead toxicity".[67]

Vitamin C has limited popularity as a treatment for autism spectrum symptoms. A 1993 study of 18 children with ASD found some symptoms reduced after treatment with vitamin C,[68] but these results have not been replicated.[69] Small clinical trials have found that vitamin C might improve the sperm count, sperm motility, and sperm morphology in infertile men[70], or improve immune function related to the prevention and treatment of age-associated diseases.[71] However, to date, no large clinical trials have verified these findings.

A preliminary study published in the Annals of Surgery found that the early administration of antioxidant supplementation using α-tocopherol and ascorbic acid reduces the incidence of organ failure and shortens ICU length of stay in this cohort of critically ill surgical patients.[72] More research on this topic is pending.

Dehydroascorbic acid, the main form of oxidized Vitamin C in the body, was shown to reduce neurological deficits and mortality following stroke, due to its ability to cross the blood-brain barrier, while "the antioxidant ascorbic acid (AA) or vitamin C does not penetrate the blood-brain barrier".[73] In this study published by the Proceedings of the National Academy of Sciences in 2001, the authors concluded that such "a pharmacological strategy to increase cerebral levels of ascorbate in stroke has tremendous potential to represent the timely translation of basic research into a relevant therapy for thromboembolic stroke in humans". No such "relevant therapies" are available yet and no clinical trials have been planned.

In January 2007 the US Food and Drug Administration approved a Phase I toxicity trial to determine the safe dosage of intravenous vitamin C as a possible cancer treatment for "patients who have exhausted all other conventional treatment options."[74] Additional studies over several years would be needed to demonstrate whether it is effective.[75]

In February 2007, an uncontrolled study of 39 terminal cancer patients showed that, on subjective questionnaires, patients reported an improvement in health, cancer symptoms, and daily function after administration of high-dose intravenous vitamin C.[76] The authors concluded that "Although there is still controversy regarding anticancer effects of vitamin C, the use of vitamin C is considered a safe and effective therapy to improve the quality of life of terminal cancer patients".

It has been observed that while serum or blood plasma levels follow the circadian rhythm or short term dietary changes, those within tissues themselves are more stable and give a better view of the availability of ascorbate within the organism. However, very few hospital laboratories are adequately equipped and trained to carry out such detailed analyses, and require samples to be analyzed in specialized laboratories.[77][78]

Adverse effects

While being harmless in most typical quantities, as with all substances to which the human body is exposed, vitamin C can still cause harm under certain conditions.

Common side-effects

Relatively large doses of vitamin C may cause indigestion, particularly when taken on an empty stomach.

When taken in large doses, vitamin C causes diarrhea. In one trial, doses up to 6 grams of ascorbic acid were given to 29 infants, 93 children of preschool and school age, and 20 adults for more than 1400 days. With the higher doses, toxic manifestations were observed in five adults and four infants. The signs and symptoms in adults were nausea, vomiting, diarrhea, flushing of the face, headache, fatigue and disturbed sleep. The main toxic reactions in the infants were skin rashes.[79]

For decades, large doses of vitamin C have been speculated to trigger oxalate formation and increase absorption of dietary oxalate, possibly causing kidney stones.[82] However, this speculation may not be justified since there is no clear relationship between excess ascorbic acid intake and kidney stone formation. [83]

During the first month of pregnancy, high doses of vitamin C may suppress the production of progesterone from the corpus luteum.[84] Progesterone, necessary for the maintenance of a pregnancy, is produced by the corpus luteum for the first few weeks, until the placenta is developed enough to produce its own source. By blocking this function of the corpus luteum, high doses of vitamin C (1000+ mg) is theorized to induce an early miscarriage. In a group of spontaneously aborting women at the end of the first trimester, the mean values of vitamin C were significantly higher in the aborting group. However, the authors point out that this relationship may not necessarily be a causal one.[85]

Chance of overdose

As discussed previously, vitamin C exhibits remarkably low toxicity. The LD50 (the dose that will kill 50% of a population) in rats is generally accepted to be 11.9 grams per kilogram when taken orally.[33] The LD50 in humans remains unknown, owing to medical ethics that preclude experiments which would put patients at risk of harm. However, as with all substances tested in this way, the LD50 is taken as a guide to its toxicity in humans and no data to contradict this has been found.

Natural and artificial dietary sources

Rose hips are a particularly rich source of vitamin C

The richest natural sources are fruits and vegetables, and of those, the camu camu fruit and the Kakadu plum contain the highest concentration of the vitamin. It is also present in some cuts of meat, especially liver. Vitamin C is the most widely taken nutritional supplement and is available in a variety of forms, including tablets, drink mixes, crystals in capsules or naked crystals.

Vitamin C is absorbed by the intestines using a sodium-ion dependent channel. It is transported through the intestine via both glucose-sensitive and glucose-insensitive mechanisms. The presence of large quantities of sugar either in the intestines or in the blood can slow absorption.[86]

Plant sources

While plants are generally a good source of vitamin C, the amount in foods of plant origin depends on: the precise variety of the plant, the soil condition, the climate in which it grew, the length of time since it was picked, the storage conditions, and the method of preparation.[87]

The following table is approximate and shows the relative abundance in different raw plant sources.[88][89][90] As some plants were analyzed fresh while others were dried (thus, artifactually increasing concentration of individual constituents like vitamin C), the data are subject to potential variation and difficulties for comparison. The amount is given in milligrams per 100 grams of fruit or vegetable and is a rounded average from multiple authoritative sources:

Plant source

Amount (mg / 100g)

Kakadu plum

3100

Camu Camu

2800

Rose hip

2000

Acerola

1600

Amla

720

Seabuckthorn

695

Jujube

500

Baobab

400

Blackcurrant

200

Red pepper

190

Parsley

130

Guava

100

Kiwifruit

90

Broccoli

90

Loganberry

80

Redcurrant

80

Brussels sprouts

80

Wolfberry (Goji)

73 ^

Lychee

70

Cloudberry

60

Persimmon

60

^ average of 3 sources; dried

Plant source

Amount (mg / 100g)

Papaya

60

Strawberry

60

Orange

50

Lemon

40

Melon, cantaloupe

40

Cauliflower

40

Grapefruit

30

Raspberry

30

Tangerine

30

Mandarin orange

30

Passion fruit

30

Spinach

30

Cabbage raw green

30

Lime

20

Mango

20

Potato

20

Melon, honeydew

20

Mango

16

Cranberry

13

Tomato

10

Blueberry

10

Pineapple

10

Plant source

Amount (mg / 100g)

Pawpaw

10

Grape

10

Apricot

10

Plum

10

Watermelon

10

Banana

9

Carrot

9

Avocado

8

Crabapple

8

Cherry

7

Peach

7

Apple

6

Blackberry

6

Beetroot

5

Pear

4

Lettuce

4

Cucumber

3

Eggplant

2

Fig

2

Bilberry

1

Horned melon

0.5

Medlar

0.3

Animal sources

Goats, like almost all animals, make their own vitamin C. An adult goat will manufacture more than 13,000 mg of vitamin C per day in normal health and levels manyfold higher when faced with stress.

The overwhelming majority of species of animals and plants synthesise their own vitamin C, making some, but not all, animal products, sources of dietary vitamin C.

Vitamin C is most present in the liver and least present in the muscle. Since muscle provides the majority of meat consumed in the western human diet, animal products are not a reliable source of the vitamin. Vitamin C is present in mother's milk and, in lower amounts, in raw cow's milk, with pasteurized milk containing only trace amounts.[91] All excess Vitamin C is disposed of through the urinary system.

The following table shows the relative abundance of vitamin C in various foods of animal origin, given in milligram of vitamin C per 100 grams of food:

Food

Amount (mg / 100g)

Calf liver (raw)

36

Beef liver (raw)

31

Oysters (raw)

30

Cod roe (fried)

26

Pork liver (raw)

23

Lamb brain (boiled)

17

Chicken liver (fried)

13

Lamb liver (fried)

12

Lamb heart (roast)

11

Food

Amount (mg / 100g)

Lamb tongue (stewed)

6

Human milk (fresh)

4

Goat milk (fresh)

2

Cow milk (fresh)

2

Beef steak (fried)

0

Hen's egg (raw)

0

Pork bacon (fried)

0

Calf veal cutlet (fried)

0

Chicken leg (roast)

0

Food preparation

Vitamin C chemically decomposes under certain conditions, many of which may occur during the cooking of food. Normally, boiling water at 100°C is not hot enough to cause any significant destruction of the nutrient, which only decomposes at 190°C,
[33] despite popular opinion. However, pressure cooking, roasting, frying and grilling food is more likely to reach the decomposition temperature of vitamin C. Longer cooking times also add to this effect, as will copper food vessels, which catalyse the decomposition.[33]

Another cause of vitamin C being lost from food is leaching, where the water-soluble vitamin dissolves into the cooking water, which is later poured away and not consumed. However, vitamin C doesn't leach in all vegetables at the same rate; research shows broccoli seems to retain more than any other.[92] Research has also shown that fresh-cut fruit don't lose significant nutrients when stored in the refrigerator for a few days.[93]

Vitamin C supplements

Vitamin C is widely available in the form of tablets and powders. The Redoxon brand, launched in 1934 by Hoffmann-La Roche, was the first mass-produced synthetic vitamin C.

Vitamin C is the most widely taken dietary supplement.[94] It is available in many forms including caplets, tablets, capsules, drink mix packets, in multi-vitamin formulations, in multiple antioxidant formulations, and crystalline powder. Timed release versions are available, as are formulations containing bioflavonoids such as quercetin, hesperidin and rutin. Tablet and capsule sizes range from 25 mg to 1500 mg. Vitamin C (as ascorbic acid) crystals are typically available in bottles containing 300 g to 1 kg of powder (a teaspoon of vitamin C crystals equals 5,000 mg).

Artificial modes of synthesis

Vitamin C is produced from glucose by two main routes. The Reichstein process, developed in the 1930s, uses a single pre-fermentation followed by a purely chemical route. The modern two-step fermentation process, originally developed in China in the 1960s, uses additional fermentation to replace part of the later chemical stages. Both processes yield approximately 60% vitamin C from the glucose feed.[95]

Research is underway at the Scottish Crop Research Institute in the interest of creating a strain of yeast that can synthesise vitamin C in a single fermentation step from galactose, a technology expected to reduce manufacturing costs considerably.[27]

World production of synthesised vitamin C is currently estimated at approximately 110,000 tonnes annually.
Main producers today are BASF/Takeda, DSM, Merck and the China Pharmaceutical Group Ltd. of the People's Republic of China. China is slowly becoming the major world supplier as its prices undercut those of the US and European manufacturers.[96]

"Plasma vitamin C concentrations are maintained between 10 and 160 μM, and any excess of the vitamin is excreted by the kidney. In contrast, the vitamin is concentrated to at least 100 times the plasma level in specific tissues such as the adrenal glands, pituitary gland, thymus, retina, corpus luteum, and a variety of neuronal cell types."

↑Stone, Irwin (July 16, 1978). Eight Decades of Scurvy. The Case History of a Misleading Dietary Hypothesis. Retrieved on 2007-04-06. “Biochemical research in the 1950’s showed that the lesion in scurvy is the absence of the enzyme, L-Gulonolactone oxidase (GLO) in the human liver (Burns, 1959). This enzyme is the last enzyme in a series of four which converts blood sugar, glucose, into ascorbate in the mammalian liver. This liver metabolite, ascorbate, is produced in an unstressed goat for instance, at the rate of about 13,000 mg per day per 150 pounds body weight (Chatterjee, 1973). A mammalian feedback mechanism increases this daily ascorbate production many fold under stress (Subramanian et al., 1973)”

"Our results demonstrate that yeast cells are capable of direct fermentation of L-galactose to L-AA. However, given that L-galactose is an extremely rare and expensive sugar a process using L-galactose as a starting material could never be economical. In order to overcome this problem, we are currently developing new yeast strains with extended metabolic competence for the synthesis of L-galactose directly from inexpensive substrates."

↑Study of High-Dose Intravenous (IV) Vitamin C Treatment in Patients With Solid Tumors. Retrieved on August 2, 2007. “The primary purpose of this study is to evaluate the safety and tolerability of vitamin C (ascorbic acid) given by injection into the vein.
The second and third purpose of conducting this study is to observe any evidence of tumor response to the vitamin C and compare the level of fatigue (weakness), pain control, ability to do things, and quality of life, before and after vitamin C is given. (Phase I)”

"Serum and plasma vitamin C measurements do not correlate well with tissue levels while lymphocyte vitamin C levels provide the most accurate assessment of the true status of vitamin C stores and are not affected acutely by circadian rhythm or dietary changes."

"The plasma concentration of vitamin C is considered to be strongly correlated with transient consumption of foods. The measurement of lymphocyte vitamin C might be expected to be a more reliable antioxidant biomarker than plasma vitamin C level. In this report, we demonstrated that the lymphocyte vitamin C level is significantly lower in type 2 diabetic patients, but we could not observe such an association in plasma vitamin C levels. In diabetes, therefore, the measurement of lymphocyte vitamin C might be expected to be a more reliable antioxidant biomarker than plasma vitamin C level."